1. Field of the Invention
This invention relates to a variable capacitor capable of precisely controlling capacity variation over a wide range.
2. Description of the Prior Art
Conventionally, there has generally been used a PN junction element as a variable capacitor as shown in FIG. 1. In FIG. 1, the reference numeral 1 designates an N-type semiconductor region, 2 a P-type semiconductor region, 3 a PN junction, 4 and 5 ohmic electrodes provided in the regions 1 and 2, respectively, 6 and 7 leading terminals provided on the electrodes 4 and 5, respectively, and 8 a depletion layer.
With this arrangement, the depletion layer 8 increases or decreases in response to bias voltage applied to the leading terminals 6 and 7, whereby the variation of capacity in accordance with the increase and decrease of the depletion layer 8 is read out between the leading terminals 6 and 7.
However, the conventional variable capacitor utilizing such a PN junction element has had the following drawbacks:
(1) Due to the fact that the conventional variable capacitor makes use of the fact that the increase or decrease of the depletion layer at the PN junction is dependent on the bias voltage, the minimum capacity is determined by impurity concentration in the semiconductor regions while the maximum capacity is determined by the increase of the conductance component. Therefore, it is practically impossible to permit a large variable range of the capacity when the Q factor is large. Additionally, the larger the capacity variation the higher the Q factor becomes. Therefore, the conventional variable capacitor is attended with difficulties in designing the circuit.
(2) Due to the fact that supply of the bias voltage for varying capacity and reading of the capacity variation are performed by means of the common leading terminals, the capacitor is apt to cause undesired capacity variation in response to voltage of the input signal itself when the capacitor is adopted in a resonance circuit, etc., resulting in signal deterioration. Further, since there is required a specific circuit arrangement in which interference between the input signal voltage and the bias voltage is small, the conventional variable capacitor is restricted to a few uses.
(3) The impurity concentration in the semiconductor regions for determining the capacity of the depletion layer is controlled by a control means such as diffusion, ion implantation, etc. However, since such means cannot realize a good yield, integration in an IC circuit is practically impossible.